Simplifying solar-based battery charging

May 06, 2014 //By Steve Knoth and Albert Wu
Simplifying solar-based battery charging
Steve Knoth, Senior Product Marketing Engineer, Power Products Group and Albert Wu, Design Manager, Power Products Group, Linear Technology Corporation explains how a buck-boost solar powered battery charging controller can offer an efficient solar-powered solution.

Background

Solar power is green and abundantly 'free', but can often be less than reliable.  Varying temperature effects that shift the solar panel’s optimal power delivery point, in addition to device aging, partial shading, the sun going down, animal waste, etc. can all impede a panel’s performance. Due to these reliability and variability concerns, nearly all solar-powered devices feature rechargeable batteries for backup power purposes.

Once just lead-acid based, these batteries have now expanded to include lithium-based chemistries too. The goal of the solar-based recharging system is to extract as much of the solar power as possible to charge the batteries quickly, as well as maintaining their state of charge. Furthermore, drain on the battery when the panel is lightly, or not illuminated, is important and should be minimized whenever possible.

Clearly, solar powered applications are on the rise. Solar panels of various sizes now power a variety of innovative applications from crosswalk marker lights to trash compactors to marine buoy lights. Some batteries used in solar powered applications are a type of deep cycle battery capable of surviving prolonged, repeated charge cycles, in addition to deep discharges.  These type of batteries are commonly found in 'off grid' (i.e., disconnected from the electric utility company) renewable energy systems such as solar or wind power generation. System up time is paramount for off-grid installations due to proximity access difficulties.

Solar panel basics

For a given amount of light energy and operating conditions, a solar panel has a certain output voltage at which peak output power is produced.  Figure 1 shows the characteristics of a 72 cell panel at a panel temperature of 60°C.  The blue line shows the I-V curve of the panel with the x-axis being the panel voltage.  The dashed red line shows the resulting output power of the panel as the panel voltage is swept from 0 V to the open circuit voltage of the panel using a

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